Sanofi disclosed that its Bruton’s tyrosine kinase inhibitor tolebrutinib failed the Phase III Perseus study in primary progressive multiple sclerosis (PPMS), and the company said the drug’s U.S. regulatory review in non‑relapsing secondary progressive MS (nrSPMS) will likely be delayed beyond the targeted PDUFA date. The Perseus miss came after the drug was acquired in a multi‑billion dollar deal and represented a major setback for Sanofi’s MS strategy. Sanofi said it submitted additional materials to regulators, filed an expanded access protocol, and will reassess its development plan. Company statements stressed confidence in the agent’s profile in other MS subtypes while acknowledging the clinical and regulatory implications of the Perseus readout. Investors and competitors reacted to the dual negative signals: a late‑stage clinical failure in one MS population and slowed regulatory timing in another. The combined developments will force program reprioritization and could alter expectations for Sanofi’s neurology pipeline.